1. Field of the Invention
The present invention relates to photoelectrodes for photoelectrochemical cells. More particularly, this invention relates to photoelectrodes composed of a surface-chalcogenated molybdenum or tungsten metal.
2. Description of the Prior Art
Recently, considerable interest has been aroused in solar energy as well as in coal and nuclear energy as substitutive energy sources for petroleum. One can cite electric power generation by solar cells as one of the methods of utilizing solar energy. However, solar cells are so expensive that their use has been quite limited. Drastic cost reduction is one of the essential conditions for solar cells to be widely used as an economical means of solar energy utilization.
In recent years, considerable research efforts have been focused on the photoelectrochemical cells (PEC), because they can be regarded as one of the candidates for economical solar cells. In the PEC, a junction between the electrolyte solution and a semiconductor photoelectrode is used as the origin of photovoltage. This liquid-semiconductor junction can be formed by inserting the semiconductor electrode in an electrolyte solution. Therefore, the junction formation is very easy in the PEC as compared with a solid state p-n junction which can be formed only by using complicated technology such as thermal diffusion, ion implantation, or co-evaporation of dopant atoms. In addition, polycrystalline semi-conductors are as effective as, or in some cases more effective than, expensive single crystal semiconductors in PEC's. These factors have led to the expectation that the PEC would be more economical than dry-type solar cells.
The most important device in the PEC is the photoelectrode which has the ability of converting photons to electricity. In order to prepare practical PEC's, the photoelectrode must be (1) highly efficient in solar energy conversion, (2) long-lived, and (3) inexpensive and easy to produce.
One of the major problems with the semiconducting materials heretofore employed in the PEC (GaP, GaAs, CdS, CdSe, CdTe) is the gradual photocorrosion which causes a rapid drop in energy conversion efficiency. Recently, it was reported that electrolytes containing polychalcogenides such as polysulfides, polyselenides and polytellurides have some stabilizing effect [A. Ellis, S. Kaiser, J. Bolts and M. Wrighton, Journal of the American Chemical Society 99, 2839, (1977)]. However, the electrolyte solutions also present problems because they are strongly alkaline and corrosive to cell wall materials, requiring the use of expensive corrosion resistant materials. There are also environmental problems since the solutions are highly toxic.
On the other hand, single crystals of chalcogenides of molybdenum or tungsten such as MoS.sub.2, MoSe.sub.2 and WSe.sub.2 have been reported to be effective as photoelectrodes for PEC's and to be free of photocorrosion [H. Tributsch, Berichte der Bunsen-Gesellschaft fur physikalische Chemie 81, 361 (1977); ibid., 82, 169, 1331 (1978)]. However, these single crystals are not adequate as practical photoelectrodes for PEC's because of difficulties and high cost of preparing them.